Settings: snap guide line width, user-docs sync checklist, LF source endings

[trailcode]

Summary

Adds Snap guide line width in Settings, completes user-guide coverage for snap-related gui JSON keys, introduces an agent doc-sync checklist, and standardizes LF line endings for project-owned source.

Closes #126.

Changes

Snap guide line width

• gui.snap_guide_line_width (default 1.0, range 0.5–8.0) in Settings → Sketch.
• Drives OCCT SetWidth for snap axis guides, markers, and co-axial overlay.
• Persisted in ezycad_settings.json; applied on create and redisplay.

User documentation

• docs/usage-settings.md — Settings pane list, Options cross-links, gui JSON table (snap_guide_*, annotate_all_coaxial_nodes, permanent_node_anno_scale).
• docs/usage-sketch.md — sketch snapping intro updated for line width.

Agent / contributor workflow

• agents/user-docs-sync.md — checklist for syncing user docs when adding or changing user-visible functionality.
• Linked from agents/README.md, agents/dev.md, docs/ezycad_doc_style.md.

Line endings

• .gitattributes — LF for src/, tests/, scripts/, docs/, etc.; binary overrides for images.
• docs/ezycad_code_style.md — line-ending convention documented.

Commits

Commit	Description
facc82e	Snap guide line width setting + user guide JSON/docs
7b88b9f	Agent user-docs-sync checklist + LF gitattributes

Test plan

• Build EzyCad_lib in Release.
• Settings → Sketch → Snap guide line width — guides thicken/thin on next snap.
• Restart app — width persists.
• usage-settings.md JSON table matches gui keys in saved settings.
• sphinx-build -b html -W docs docs/_build passes.

Compare

main...Trailcode/settings